1. Field of the Invention
The invention relates to interference cancellation in a receiver of a radio system.
2. Description of the Related Art
Mobile communication systems and the number of users being on the increase, the mobile communication systems must increasingly often be located near each other's frequency bands, i.e. guard bands between the systems must be narrowed down. The inevitable result is that interferences caused by radio systems to each other will be on the increase, which should be taken into account when designing the systems. The situation can be illustrated by FIG. 1, wherein two radio systems operate within the same geographical area. A base transceiver station 100 belongs to a first radio system and serves a terminal device 104. A base transceiver station 102, in turn, belongs to a second radio system and serves a terminal device 106. A terminal device 106 residing in the coverage area 110 of the base transceiver station 100 is far away from the base transceiver station 102 serving it, which means that it is compelled to use a high transmission power. The transmission of the terminal 104 using a low transmission power experiences significant interference caused by the transmission of the terminal device 106 belonging to the second system.
External interference, such as signal energy leaking from a neighbouring system, is cancelled in broadband receivers e.g. by using FFT (Fast Fourier Transformation), TFD (Time Frequency Distribution) or an adaptive filter. Known interference cancellation solutions often require that an interference signal should be easily detectable. The performance of external interference cancellation algorithms is usually sufficient if the ISR (Interference to Signal Ratio) is of the order of 20 dB or the SNR (Signal to Noise Ratio) is of the order of 10 to 20 dB. In many operating situations, however, the external interference is weak, the interference to signal ratio being of the order of 1 dB, in which case the performance of the interference cancellation algorithms intended for cancelling external interference is very poor. In addition, in many operation situations noise is strong, which makes a signal difficult to detect from among the noise.